Hitherto, as a large capacity inverter circuit configured to drive a motor of a compressor of an air-conditioning apparatus, a circuit employing a method in which a three-phase full-wave rectifier circuit generates a DC voltage for driving the inverter is used. In this air-conditioning apparatus, for the purpose of enhancing energy consumption efficiency during a rated cooling/heating operation (coefficient of performance (COP)) or year-round energy consumption efficiency (annual performance factor (APF)), there has hitherto been a tendency that the motor for the compressor is designed so that an induced voltage (counter electromotive force) at about a rotation speed used in rated operation of the air-conditioning apparatus is similar to the commercial power supply. In this case, when it is attempted to operate the inverter circuit in a high rotation speed region exceeding the rotation speed (for example, in overload operation), the inverter circuit cannot output a voltage that is substantially equal to or higher than the commercial power supply, and thus, a current passing through the motor of the compressor increases due to output voltage saturation, which lowers motor efficiency and increases loss in the inverter circuit, resulting in lowered efficiency of the power conversion apparatus. Further, constraints on demagnetization resistance of a magnet used in the motor for the compressor and a permissible current and a permissible temperature of a semiconductor used in the inverter circuit may narrow the operating range.
In order to address those problems, for the purpose of attaining high efficiency in driving the motor and expanding the operating range, there has been proposed a power conversion apparatus including a booster circuit configured to expand the range of an output voltage of an inverter circuit (see, for example, Patent Literature 1).
In the power conversion apparatus described in Patent Literature 1, the booster circuit including a reactor, a backflow prevention diode, and a switching device is arranged between a rectifier circuit and the inverter circuit. A DC voltage rectified by the rectifier circuit is boosted by the booster circuit.
In the booster circuit, energy is accumulated in the reactor during an ON period of the switching device, and the accumulated energy is released during an OFF period thereof to boost the DC voltage. The DC voltage from the booster circuit is controlled by a time period during which the switching device is ON (on-duty). Through control of the on-duty of the switching device of the booster circuit, the voltage applied to the motor is increased to enable improved efficiency through reducing a motor current and expansion of the operating range of the motor.